This invention relates to a glass-ceramic substrate for an information storage medium, a method for manufacturing the same and an information storage disk. In this specification, the term xe2x80x9cinformation storage mediumxe2x80x9d includes stationary type hard disks, removable type hard disks and card type hard disks used respectively such as so-called hard disks for personal computers and other information storage medium in the form of a disk which can be used for storage of data and can be also used in digital video cameras and digital cameras.
Aluminum alloy has been widely used as a material of a magnetic disk substrate used in a stationary type magnetic information storage device such as a hard disk of a computer. In this case, a blank substrate made of an aluminum alloy having a surface roughness Ra of about 200 xc3x85 to 1000 xc3x85 is subjected to degreasing, etching, substitution of Zn or Sn and Pd, electroless Nixe2x80x94P plating, annealing, wrapping and polishing, cleaning and testing and forming of a Cr undercoat layer, a magnetic film and a protective layer to thereby form a magnetic information storage disk. Since, however, the aluminum alloy is a soft material, its Young""s modulus and surface hardness are low with the result that a significant degree of vibration takes place during rotation of the disk drive at a high speed rotation which leads to deformation of the disk and hence it is difficult to cope with the requirement for making the disk thinner. Thus, the aluminum alloy cannot cope sufficiently with requirement for high-speed driving with a high recording density.
As a material for overcoming the above problems of the aluminum alloy substrate, known in the art are glass-ceramics and chemically tempered glass such as alumino-silicate glasses. Currently used glass-ceramic substrates and chemically tempered glass substrates do not have Nixe2x80x94P plating on their surface as in the aluminum alloy substrates but the Cr undercoat layer and magnetic film are directly formed on the surface of the substrates. This is because Nixe2x80x94P plating having excellent adhesion cannot be achieved on the surface of the glass-ceramic substrates and the chemically tempered glass substrates.
However, in the case that the Cr undercoat layer and magnetic film are formed directly on the surface of a glass-ceramic substrate or a chemically tempered glass substrate without the Nixe2x80x94P plating, magnetic substance grains of the magnetic film tend to have an excessively large grain diameter as compared with a case where the Cr undercoat layer and magnetic film are formed on the Nixe2x80x94P plating and this will become an obstacle to the requirement for the high density recording in the future.
Examples in which the Nixe2x80x94P plating is applied to a glass-ceramic substrate or a chemically tempered glass substrate are disclosed in, for example, Japanese Patent Application Laid-open Publication No. Sho 62-230651, Japanese Patent Application Laid-open Publication No. Sho 63-225915, Japanese Patent Application Laid-open Publication No. Hei 6-87680, Japanese Patent Application Laid-open Publication No. Hei 7-272263. In these examples, chemical etching is made on the substrate in an attempt to roughen its surface and thereafter the Nixe2x80x94P plating is applied to the roughened surface. Despite these efforts, however, sufficient roughening of the surface of the substrate by etching could not be achieved in most of these glass-ceramic substrates and chemically tempered glass substrates which resulted in insufficient adhesion of the Nixe2x80x94P layer. In some glass-ceramic substrates, adhesion of the Nixe2x80x94P layer was improved at the sacrifice of flatness of the surface and these substrates are not proper as a high recording density information storage medium required today. Thus, there is no practicable technique today by which the Nixe2x80x94P plating can be sufficiently applied to glass-ceramic substrates and chemically tempered glass substrates.
As a type of a chemically tempered glass, known in the art are alumino-silicate glasses (SiO2xe2x80x94Al2O3xe2x80x94Na2O) disclosed in Japanese Patent Application Laid-open Publication No. Hei 8-48537 and Japanese Patent Application Laid-open Publication No. Hei 5-32431. This material, however, has the following disadvantages:
(1) Since polishing is made after the chemical tempering process, the chemically tempered layer is seriously instable in making the disk thinner. Further, the chemically tempered layer causes change with time after use for a long period of time with resulting deterioration in the magnetic property of the disk.
(2) Since the chemically tempered glass contains Na2O and K2O as its essential ingredients, the film forming property of the glass is adversely affected, and a barrier coating over the entire surface of the glass becomes necessary for preventing elution of Na2O and K2O ingredients and this prevents stable production of the product at a competitive cost.
(3) The chemical tempering is made for improving mechanical strength of the glass but this basically utilizes strengthening stress within the surface layer and the inside layer of the glass and hence its Young""s modulus is about 83 GPa or below which is about equivalent to ordinary amorphous glass. Therefore, use of the glass for a disk with a high speed rotation is limited.
(4) In applying Nixe2x80x94P plating, etching is made by using an HF type solution. Since etching is made uniformly over the surface of the substrate in a chemically tempered glass, a microstructure for securing adhesion of the Nixe2x80x94P plating cannot be obtained with the result that a sufficient substrate for the Nixe2x80x94P plating cannot be obtained. Therefore, characteristics of the glass for a high recording density information storage medium are not sufficient.
Aside from the aluminum alloy substrate and chemically tempered glass substrate, known in the art are some glass-ceramic substrates. For example, Japanese Patent Application Laid-open No. Hei 9-35234 and EP0781731A1 disclose glass-ceramic substrates for a magnetic disk made of a Li2Oxe2x80x94SiO2 system composition and has crystal phases of lithium disilicate and xcex2-spodumene, or crystal phases of lithium disilicate and xcex2-cristobalite. In these publications, however, the relation between Young""s modulus and specific gravity with respect to high speed rotation is not taken into consideration at all. Further, no suggestion is made about an HF etching technique suitable for Nixe2x80x94P plating and conditions of a microstructure suitable for Nixe2x80x94P plating.
Japanese Patent Application Laid-Open Publication No. Hei 9-77531 discloses a glass-ceramic of a SiO2xe2x80x94Al2O3xe2x80x94MgOxe2x80x94ZnOxe2x80x94TiO2 system which has Young""s modulus of 93.4 GPa to 160.11 GPa and volume density of 2.83 to 3.24 and also a substrate for a high rigidity magnetic information storage disk made of this glass-ceramic. This glass-ceramic contains a large amount of Spinel crystal ((Mg or Zn) Al2O4) as its predominant crystal phase and MgTi2O5 or Zn2Ti2O5 and several other crystals as its subordinate crystal phases. The glass-ceramic contains a large amount of Al2O3 and has a high specific gravity and a high Young""s modulus.
Addition of such a large amount of Al2O3, however, is undesirable from the standpoint of production because it causes deterioration of melting property of the base glass and also deterioration of resistance to devitrification. Further, in this publication, the relation of Young""s modulus (GPa)/specific gravity and the value of specific gravity per se which are necessary for the high speed rotation are not taken into consideration at all. No suggestion is made either as to an HF technique suitable for Nixe2x80x94P plating and conditions of microstructure for Nixe2x80x94P plating. Particularly, the specific gravity in this publication is a high value of 2.83 or over. This publication therefore merely proposes a glass-ceramic substrate made of a rigid material. Moreover, the glass-ceramic of this system has the serious disadvantage that processability is poor and therefore is not suited for a large scale production since it is too hard and hence the improvement of this glass-ceramic as a high recording density information storage medium substrate is still not sufficient.
It is, therefore, an object of the invention to eliminate the above described disadvantages of the prior art materials and provide a glass-ceramic substrate for an information storage medium capable of coping with the tendency toward a high speed driving, high recording density recording, namely having excellent adhesion to Nixe2x80x94P plating.
It is another object of the invention to provide a method for manufacturing such glass-ceramic substrate.
It is another object of the invention to provide an information storage medium made by applying Nixe2x80x94P plating and forming a film of an information storage medium on this glass-ceramic substrate.
Accumulated studies and experiments made by the inventor of the present invention for achieving the above described objects of the invention have resulted in the finding, which has led to the present invention, that a glass-ceramic substrate made of a glass-ceramic having a specific microstructure in the surface portion thereof exhibits an excellent adhesion characteristic in Nixe2x80x94P plating.
For achieving the object of the invention, there is provided a glass-ceramic substrate for an information storage medium made of a glass-ceramic having a crystal phase dispersed in a glass matrix wherein a ratio of the crystal phase (the amount of crystal) in the glass-ceramic is within a range from 50 wt % to 70 wt % and crystal grains having an average crystal grain diameter within a range from 0.10 xcexcm to 0.50 xcexcm are exposed on the surface of the substrate.
In one aspect of the invention, there is provided a glass-ceramic substrate for an information storage medium made of a glass-ceramic having a crystal phase dispersed in a glass matrix wherein a ratio of Young""s modulus to specific gravity is 39 GPa or over and crystal grains having an average crystal grain diameter within a range from 0.10 xcexcm to 0.50 xcexcm are exposed on the surface of the substrate.
In another aspect of the invention, there is provided a glass-ceramic substrate for an information storage medium made of a glass-ceramic having a crystal phase dispersed in a glass matrix wherein crystal grains having an average crystal grain diameter within a range from 0.10 xcexcm to 0.50 xcexcm are present in a surface portion of the substrate and the substrate has a surface microstructure in which respective crystal grains present in the surface portion of the substrate are fixed to the surface portion with half or more in volume of the respective crystal grains being exposed above the surface of the glass matrix.
In another aspect of the invention, the glass-ceramic substrate has Young""s modulus within a range from 95 GPa to 160 GPa, specific gravity within a range from 2.40 to 2.80 and a ratio of Young""s modulus to specific gravity within a range from 39 GPa to 57 GPa.
In another aspect of the invention, the glass-ceramic substrate is made by applying etching to a glass-ceramic and has a surface roughness Ra (arithmetic average roughness) after polishing of 500 xc3x85 or below.
In another aspect of the invention, the glass-ceramic substrate is made by applying etching to the glass-ceramic soaked in a 4 wt % to 30 wt % HF type etching solution at an etching rate of 30 xc3x85/second or over.
In another aspect of the invention, the surface roughness (Ra) of the glass-ceramic substrate is within a range from 50 xc3x85 to 10000 xc3x85.
In another aspect of the invention, said glass-ceramic contains, as its predominant crystal phase or phases, at least one crystal selected from the group consisting of cordierite (Mg2Al4Si5O18), cordierite solid solution (Mg2Al4Si5O18 solid solution), Spinel ((Mg or Zn)(Al or Ti)2O4), Spinel solid solution ((Mg or Zn)(Al or Ti)2O4), enstatite (MgSiO3), enstatite solid solution (MgSiO3 solid solution), quartz (SiO2) and quartz solid solution (SiO2).
In another aspect of the invention, crystal grains of the predominant crystal phase or phases of the glass-ceramic have an average crystal grain diameter within a range from 0.05 xcexcm to 1.0 xcexcm.
In another aspect of the invention, the glass-ceramic contains a crystal phase of cordierite having an average crystal grain diameter within a range from 0.10 xcexcm to 1.0 xcexcm.
In another aspect of the invention, the glass-ceramic contains a crystal phase of enstatite having an average crystal grain diameter within a range from 0.10 xcexcm to 1.0 xcexcm.
In another aspect of the invention, the glass-ceramic contains acicular, granular or sheet-shaped crystal grains having an average crystal grain diameter within a range from 0.10 xcexcm to 1.0 xcexcm.
In another aspect of the invention, a coefficient of thermal expansion within a temperature range from xe2x88x9250xc2x0 C. to +70xc2x0 C. is within a range from 30xc3x9710xe2x88x927/xc2x0 C. to 50xc3x9710xe2x88x927/xc2x0 C.
In another aspect of the invention, the glass-ceramic has a composition which comprises, in weight percent:
In another aspect of the invention, the glass-ceramic contains, as its predominant crystal phases, lithium disilicate (Li2O.2SiO2) and further contains quartz (SiO2) or quarts solid solution (SiO2 solid solution), or cristobalite (SiO2) or cristobahte solid solution (SiO2 solid solution).
In another aspect of the invention, the respective predominant crystal phases have an average crystal grain diameter within a range from 0.05 xcexcm to 0.40 xcexcm.
In another aspect of the invention, the glass-ceramic contains crystal grains of quarts or quarts solid solution having an average crystal grain diameter within a range from 0.15 xcexcm to 0.4 xcexcm and the amount of crystal of said crystal phase is within a range from 10 wt % to 70 wt %.
In another aspect of the invention, the glass-ceramic contains crystal grains of lithium disilicate having an average crystal grain diameter within a range from 0.10 xcexcm to 0.4 xcexcm and the amount of crystal of said crystal phase is within a range from 40 wt % to 70 wt %.
In another aspect of the invention, crystal grains of the glass-ceramic are substantially of a spherical shape.
In another aspect of the invention, a coefficient of thermal expansion within a temperature range from xe2x88x9250xc2x0 C. to +70xc2x0 C. is within a range from 70xc3x9710xe2x88x927/xc2x0 C. to 130xc3x9710xe2x88x927/xc2x0 C.
In another aspect of the invention, the glass-ceramic has a composition which comprises, in weight percent:
In another aspect of the invention, the glass-ceramic substrate for an information storage medium is formed by applying etching on the above described glass-ceramic substrate.
In another aspect of the invention, there is provided an information storage disk having a film of an information storage medium formed on the above described glass-ceramic substrate for an information storage medium.
In another aspect of the invention, the film of an information storage medium is a magnetic metal film.
In another aspect of the invention, there is provided a method for manufacturing a glass-ceramic substrate for an information storage medium comprising steps of melting glass raw materials, forming a glass and annealing the formed glass and thereafter heat treating the formed glass for crystallization under nucleation temperature within a range from 450xc2x0 C. to 850xc2x0 C. and crystallization temperature within a range from 740xc2x0 C. to 1000xc2x0 C.
In still another aspect of the invention, there is provided a method for manufacturing a glass-ceramic substrate for an information storage medium as defined in any of claims 1 to 22 comprising steps of soaking the glass-ceramic in 4 wt % to 30 wt % HF type etching solution and applying a chemical etching at an etching rate of 30 xc3x85/second or over.